Maintaining postural balance is crucial to mobility. Mobility disability is predictive of more severe disablement, including falls, hip fractures, recurrent hospitalizations and institutionalization. Approximately half of persons 65 years or older report a fall annually and $10 billion is spent for the care of hip fractures each year. An important national health goal is to "help individuals of all ages increase life expectancy and improve their quality of life." Identifying modifiable health factors that can delay the onset or decrease the slope of functional decline relating to balance is of key importance. The purpose of this study is to determine the association of knee extensor strength, body composition, and physical activity with balance in healthy adults. This study is a substudy of a NASA/National Institute on Aging (NIA) study. Specific questions are: 1). Is there a difference between same-age gender groups in balance? 2). Will knee strength predict balance after accounting for body composition and physical activity? 3). Do knee extensor strength, body composition and physical activity decrease the association of age with balance? 4). Does physical activity have an independent contribution to balance? Using a cross-sectional design, 180 healthy men and women from 20-90 years of age belonging to the Baltimore Longitudinal Study of Aging (BLSA) will undergo evaluations of balance using the modified "Equitest"; knee muscle strength using the "KinCom" dynamometer; body composition using dual-energy x-ray absorptiometry (DEXA) scanning; and physical activity assessment using a questionnaire. Results using sequential regression analysis, ANOVA, and ANCOVA will determine the importance of these health factors throughout the lifespan. [unreadable] [unreadable]